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Caffè espresso, espresso, or expresso; is a concentrated coffee beverage brewed by forcing hot water under pressure through finely ground coffee. The first espresso machines were introduced at the beginning of the 20th Century, with the first patent being filed by Luigi Bezzera of Milan, Italy, in 1901, but up until the mid-1940s it was a beverage produced solely with steam pressure. The invention of the spring piston lever machine and its subsequent commercial success changed espresso into the beverage as it is known today. Espresso can have a thicker consistency than drip coffee, a higher amount of dissolved solids than drip coffee per relative volume, and a serving size that is usually measured in shots, which have traditionally been around 1 fluid ounce in size. Many of espresso's chemical components quickly degrade by oxidationEspresso: The Science of Quality (2nd Edition); Andrea Illy, Rinantonio Viani; 2005. Espresso can have crema, a reddish-brown foam that floats on the surface and is composed of vegetable oils, proteins and sugars. As a result of the pressurized brewing process, all of the flavors and chemicals in a typical cup of coffee are very concentrated. For this reason, espresso lends itself to becoming the base for other drinks, such as lattes, cappuccino, macchiato and mochas. While there can be significant variation, on a ''per-volume'' basis, espresso contains approximately three times the caffeine content of regular brewed coffee (around 50 mg per fluid ounce of espresso versus around 22 mg per ounce of brewed coffee). Compared on the basis of usual serving sizes, a 1 fluid ounce shot of espresso has about half the caffeine of a standard 6 fluid ounce cup of drip style coffee, which varies from 80 to 130 mg.

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